AP 1 - Muscular System fall 2020

The Muscular System

• Functions of the Muscular System:

  • Movement: Movement of bones, food, urine, or blood through contraction.

  • Posture: Supports the body’s position.

  • Joint Stabilization: Provides strength to joints, helping to stabilize them.

  • Generating Heat: Muscle movement aids in maintaining body temperature.

Muscle Tissue

• Types of Muscle Tissue:

  1. Skeletal Muscle Tissue:

     • Responsible for moving bones, named for its striated appearance.

     • Controlled voluntarily.

  2. Smooth Muscle Tissue:

     • Forms walls of hollow organs and blood vessels.

     • Has an unstriated appearance and is involuntary.

  3. Cardiac Muscle Tissue:

     • Found in the heart.

     • Striated and involuntary.

Functional Characteristics of Muscle Tissue

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten and generate force upon stimulation.

• Extensibility: Capacity to stretch without damaging.

• Elasticity: Ability of muscle fibers to recoil after stretching.

Organization of Skeletal Muscle

• Hierarchy of Muscle Structure:

  1. Muscle (Whole Organ): Divided into fascicles.

  2. Fascicle: A bundle of muscle fibers (cells).

  3. Muscle Fiber (Cell): Basic cellular unit of muscle.

     • Contains organelles called myofibrils.

  4. Myofibril: Contractile units composed of sarcomeres.

  5. Sarcomere: Segment of myofibril responsible for contraction, composed of myofilaments.

  6. Myofilaments: Thick and thin protein fibers that slide to produce muscle contraction.

Anatomy of Skeletal Muscle

• Connective Tissue Sheaths:

  • Endomysium: Surrounds individual muscle fibers.

  • Perimysium: Surrounds bundles of fibers called fascicles.

  • Epimysium: Dense connective tissue surrounding the entire muscle.

Muscle Attachments

• Attachment Types:

  • Origin: The more immobile attachment of a muscle.

  • Insertion: The more mobile attachment site.

  • Muscle attachments must span a joint for movement.

  • Direct Attachment: Epimysium fuses with periosteum of a bone.

  • Indirect Attachment: Epimysium extends as connective tissue (e.g., tendon).

Microscopic Anatomy of Skeletal Muscle

• Structural Features:

  • Skeletal Muscle Fibers: Long, multinucleated cells containing contractile proteins.

  • Sarcolemma: Plasma membrane of a muscle fiber.

  • Sarcoplasm: Cytoplasm of a muscle fiber.

  • Glycosomes: Store glycogen, providing energy during activity.

  • Myoglobin: Binds and stores oxygen.

Myofilaments Structure

• Thick Filaments: Comprised of myosin proteins; diameter 16μm.

  • Heads of myosin have binding sites for ATP and actin.

• Thin Filaments: Diameter 8μm; consists of actin strands and regulatory proteins such as tropomyosin and troponin.

Sarcomere Structure

• A Band: Dark region containing thick and thin filaments.

• I Band: Light region containing only thin filaments.

• H Zone: Area within A band with myosin only.

• Z Disc: Provides anchorage for thin filaments; contractile unit runs from Z disc to Z disc.

Sliding Filament Theory

• Mechanism:

  • During contraction, thick and thin filaments slide past each other, shortening the sarcomere.

• Observation: A bands remain unchanged while I bands and H zones disappear during contraction.

Muscle Contraction Sequence

1. Excitation from the nervous system at a neuromuscular junction.

2. Events of excitation-contraction coupling.

3. Cross-bridge cycling occurs resulting in the sliding of filaments.

Neuromuscular Junction

• Function: Site where a motor neuron axon meets a skeletal muscle fiber.

• Action Potential: Nerve signal triggering muscle contraction.

Action Potential Dynamics

• Phases:

  • Resting Membrane: Polarized due to ion separation.

  • Threshold: Minimum depolarization to trigger action potential.

  • Depolarization: Membrane loses polarity as Na+ enters.

  • Repolarization: Membrane regains polarity as K+ exits.

Events at the Neuromuscular Junction

• Sequence of Events:

  1. Action potential arrives.

  2. Calcium influx triggers ACh release from vesicles.

  3. ACh diffuses, binding to muscle fiber receptors, causing depolarization and potential action.

Excitation-Contraction Coupling (EC coupling)

• Process:

  1. Action potential triggers calcium release from the sarcoplasmic reticulum (SR).

  2. Calcium binds to troponin, moving tropomyosin to expose actin binding sites.

  3. Myosin heads attach to actin, initiating contraction.

  4. Calcium is pumped back into SR, leading to muscle relaxation.

Cross Bridge Cycle

• Stages:

  1. Cross Bridge Formation: Activated myosin heads attach to actin.

  2. Power Stroke: Myosin pulls thin filament.

  3. Cross Bridge Detachment: Binding of new ATP causes release of myosin from actin.

  4. Cocking: ATP breakdown returns myosin to active position.

Muscle Contraction - Motor Units

• Definition: A motor unit consists of one motor neuron and all the muscle fibers it controls.

• Recruitment: Weak contractions activate fewer motor units; stronger contractions recruit more.

Muscle Contraction - Twitch Responses

• Muscle Twitch: Response to a single action potential, reflected in a myogram:

  • Latent Period: Delay before contraction begins.

  • Contraction Phase: Muscle fibers shorten.

  • Relaxation Phase: Muscles elongate back to resting length.

Types of Muscle Contractions

• Isotonic Contractions: Change in muscle length.

  • Concentric: Muscle shortening.

  • Eccentric: Muscle lengthening, generally stronger.

• Isometric Contractions: No change in muscle length (e.g., holding a weight).

Muscle Metabolism

• Energy Sources for Contraction:

  1. Stored ATP: Brief energy storage (4-6 sec).

  2. Creatine Phosphate: Reforms ATP (15 sec).

  3. Anaerobic Respiration: Quick ATP generation without oxygen.

  4. Aerobic Respiration: Produces much ATP but requires oxygen (slow).

Factors Affecting Force of Contraction

• Recruitment: More motor units yield increased force.

• Fiber Size: Larger fibers generate more force.

• Frequency of Stimulation: Efficient tetanus increases force.

• Muscle Stretch: Slight muscle stretching optimizes filament overlap for force production.

Factors Affecting Velocity and Duration of Contraction

• **Fiber Type:

  • Slow Oxidative: Prolonged contractions, oxygen-dependent, thin, red fibers.

  • Fast Glycolytic: Short, fast contractions, oxygen-independent, thicker, white fibers.

• Load: Heavier loads slow contraction rates.

• Recruitment: More motor units lead to faster contractions.

Effect of Exercise on Muscle

• Aerobic Exercises: Increase blood supply, myoglobin, and mitochondrial content in fibers, enhancing endurance.

• Resistance Exercises: Lead to hypertrophy, increasing fiber size, connective tissue, and strength.

Smooth Muscle Characteristics

• Type: Involuntary, non-striated muscle found in walls of hollow organs.

• Organization: Sheets of muscle fibers with circular and longitudinal layers.

Smooth Muscle Fibers

• Structure: Shorter fibers (2-10μm) with a single nucleus.

• Lack of Sarcomeres: Contractions still occur via the sliding filament mechanism, utilizing gap junctions for coordinated function.

Disorders of the Muscular System

• Myasthenia Gravis: Autoimmune condition attacking ACh receptors, causing muscle weakness.

• Duchenne Muscular Dystrophy (DMD): Genetic disorder leading to progressive muscle loss, primarily affects males, with a mutation in dystrophin.